In the drilling of deep wells for the production of petroleum products, rotary drilling apparatus is typically employed. Drilling fluid, typically referred to as drilling mud, is utilized in conjunction with the well drilling apparatus to perform a number of functions. The drilling mud functions to provide a coolant and lubricant for the drill bit and to lubricate the drill stem as it rotates within the well bore. The drilling fluid also functions to flush drill cuttings that are loosened from the formation by the bit during drilling operations, thus preventing the drilling cuttings from interfering with further drilling operations. The drilling fluid also functions as a transporting medium to transport the drill cuttings to the surface where the same is separated from the drilling mud by means of mechanical separators, settling tanks or pits, etc. Another function of the drilling fluid medium is that it forms a filter cake lining for the well bore, which filter cake prevents loss of drilling fluid into certain types of earth formations. In the event certain portions of the filter cake fail to provide a proper seal at certain formations, quantities of the drilling fluid medium can flow into such formations and become lost. Obviously, drilling fluid is quite expensive and, therefore, it is desirable to insure against loss of drilling fluid from the well bore.
As the rotary drill is caused to bore through pressure bearing geological formations, it is important that the hydrostatic pressure of the drilling fluid within the well bore be sufficient to overcome the pressure within the formation, thus preventing the formation fluid from entering the well bore and causing a blowout. In many cases, the formations intersected by the well bore include structural anomalies such as cracks, fissures, voids, etc. Some earth formations are very soft or porous. In each of these cases, substantial loss of drilling fluid may occur when these types of formations are encountered by the drill bit. Since drilling fluid is pumped at high pressure through the drill stem to the bottom of the well bore being drilled, this high pressure fluid medium can flow directly into the formation, thus causing loss of circulation and depleting the drilling fluid to the extent that insufficient hydrostatic head is maintained to prevent blowouts from occurring. It is necessary when lost or diminished fluid circulation occurs that immediate steps be taken to replenish lost drilling fluid material and eliminate the cause of such lost or diminished circulation. In many cases when loss of drilling fluid is occurring, such loss does not become apparent for a number of hours. During this period a substantial quantity of drilling fluid may be lost and the cost of drilling the wells will thus be increased substantially. It is desirable to provide a suitable means for immediately identifying both sudden and gradual losses of drilling fluid to the formation being drilled in order that steps may be immediately taken to correct the cause of such loss.
During well drilling operations it is necessary to change the drilling bit under circumstances where it becomes dull or when the bit becomes ineffective due to changes in the formation being drilled. When the drill stem is removed from the well bore to change out the drilling bit, it is necessary that sufficient drilling fluid remain in the well bore to maintain a necessary hydrostatic head to prevent well blowout during this particular procedure. It is therefore important to have information immediately available that indicates the total volume of drilling fluid that is stored within the settling tanks or pits at the surface. Further, by knowing the amount of drilling fluid stored in the settling tanks or pits, operating personnel of the drilling rig will be immediately aware of the volume and thus hydrostatic head of the drilling fluid within the well bore at all times. If the hydrostatic head becomes insufficient, drilling fluid may be pumped from the settling tanks into the well bore as appropriate to the desired increase in hydrostatic head.
One of the more important aspects of the drilling fluid system is the efficiency of circulation developed by the drilling fluid pump. By knowing the amount of drilling fluid injected into and flowing out of the well during ordinary circulation, the personnel in charge of drilling are made aware of a number of characteristics of the drilling fluid circulation system. For example, pump efficiency is readily determined by inspecting changes in the volume of fluid circulation. Other information such as filling of the well bore with drilling mud during insertion or removal of the drill stem, the appearance of undesirable fluid, such as formation fluid, for example, in the drilling fluid can be identified rapidly through proper monitoring of all faces of the drilling fluid system. Well drilling personnel may also need or desire to know the required time lapse between injection of drilling fluid into the well and outflow of contaminated drilling fluid from the well. Unusual changes in the time lapse between injection and outflow can be indicative of events within the well bore that might require certain corrective activities on the part of drilling fluid personnel.